This relates to package design and more particularly to patterns of electrical interconnects that are used for high performance devices such as high speed differential signaling transceiver pairs and memory interfaces. As is known in the art, such interfaces are typically implemented using a ball grid array (BGA) or a pin grid array (PGA). A BGA is an area array of solder balls or solder bumps that is located on the surface of a package. A PGA is an area array of pins underneath a package surface. The BGA or PGA is used to connect the package to the next level of package. See, R. R. Tummala, Fundamentals of Microsystems Packaging, pp. 67, 68, 279-281, 680-682, 925 (McGraw-Hill, 2001), which is incorporated by reference herein in its entirety. For convenience, the terms “contacts” or “interconnects” will be used hereafter to refer to solder balls, solder bumps, and interconnect pins as well as similar connectors
In designing high speed differential signaling interfaces, one approach has been to locate the differential signaling pairs in the vicinity of ground contacts. FIG. 1 depicts a conventional rectilinear array 100 of rows 102 and columns 104 of contact sites 110 on a substrate 120 wherein adjacent rows and adjacent columns are each spaced apart by a distance D, which is commonly referred to as the pitch. In a conventional interface, a plurality of ground contacts 130 are located at certain of the contact sites 110 spaced apart by the distance 3D or three pitch distances. A plurality of pairs of differential contacts 140, 145 are more or less randomly distributed at other contact sites 110. In FIG. 1, each differential pair is indicated by a solid line extending between a pair of contacts 140, 145. Other contacts such as contacts 160 may provide power or other signals across the interface. Some contact sites may be empty.
As will be apparent, the distance between the contacts of the differential pairs varies and is one pitch distance D in some cases and sqrt(2)*D in other cases. In some cases, two or more differential pairs are adjacent and parallel to one another such as the pairs encircled by the dotted line A or the dotted line B. These situations are likely to increase cross-talk.
As discussed below in conjunction with FIG. 4, the performance of an interconnection pattern such as that of FIG. 1 is not optimum. The differential impedance tends to vary significantly from pair to pair and the coupling coefficient is undesirably high.